THE CARBIDES GD3MN2C6 AND TB3MN2C6

Crystals of the title compounds were obtained by arc-melting cold-pressed pellets of the elemental components, followed by annealing the reaction products in an argon atmosphere slightly below the melting point. The crystal structures of these isotypic, hexagonal carbides (P6(3)/m, Z = 2) were determined from single-crystal X-ray data; Gd3Mn2C6: a = 815.0(2) pm, c = 504.93(9) pm, R = 0.012 for 526 structure factors and 18 variable parameters; Tb3Mn2C6: a = 810.5(2) pm, c = 500.5(2) pm, R = 0.025 (225 F's, 18 variables). The carbon atoms form pairs with C-C bond distances corresponding to double bonds. The three-dimensional, polyanionic manganese carbon network contains relatively large trigonal-bipyramidal voids formed by three lanthanoid and two manganese atoms. The rationalization of chemical bonding on the basis of the 18-electron rule suggests that these voids are filled by nonbonding electrons of the adjacent manganese atoms.